With improvements in resolution of solid state imaging elements, imaging elements are used not only for conventional general purposes but also for precise industrial measurements. For example, FIG. 9 shows an imaging device for measuring the three-dimensional configuration of an object invented by F. Blais et al., which is described in "SPIE vol. 728 Optics, Illumination, and Image Sensing for Machine Vision (1986), pp. 235 to 242". In FIG. 9, reference numeral 1 designates a mask having optical windows separating incident light into two beams disposed in front of an imaging device. A lens for imaging 2 is provided behind the mask 1. A solid state imaging element 9 of high resolution is provided behind the lens 2. Reference numeral 4 designates a reference plane and reference numeral 5 designates an object.
A description is given of the operation hereinafter.
As shown in FIG. 9, an optical system is adjusted such that a point A on the reference plane 4 is focused on a point A' on the imaging element 9 such as a CCD. In this state, when the object 5 is positioned at point B, reflected light at point B is focused as double images at points b and b' (corresponding to broken lines in FIG. 9) on an imaging plane of the imaging element 9.
FIG. 11 is a diagram showing the measurement of reflected light as optical information at three points B.sub.1, B.sub.2, B.sub.3 of object 5 for explaining this measurement principle in detail. The reflected light from point B.sub.1 spaced apart from a reference plane by .DELTA.L.sub.1 is focused as double images at points b.sub.1 and b'.sub.1 on the imaging plane. Reflected light from points B.sub.2 and B.sub.3 spaced apart from the reference plane 4 by .DELTA.L.sub.2 and .DELTA.L.sub.3 (.DELTA.L.sub.1 &gt;.DELTA.L.sub.2 &gt;.DELTA.L.sub.3) are respectively focused as double images at points b.sub.2 and b'.sub.2 and points b.sub.3 and b'.sub.3 on the imaging plane. Here, the relation between the length differences in respective double images .DELTA.d.sub.1, .DELTA.d.sub.2, .DELTA.d.sub.3 is .DELTA.d.sub.1 &gt;.DELTA.d.sub.2 &gt;.DELTA.d.sub.3, and it is found that the distance from reference plane 4 to object 5 (.DELTA.L) is directly proportional to the length difference of the double images (.DELTA.d) on the imaging plane. Therefore, it is possible to obtain the three-dimensional configuration of the object by measuring the length difference .DELTA.d taking into consideration the repetition of the picture elements from image information. Thus, the distance information can be measured by an imaging device of a simple structure.
In the prior art imaging device constructed as described above, although correct information is obtained when the object 5 is positioned on the same side of the references plane 4 as the imaging element 9, incorrect information results when the object 5 is positioned on the side of reference plane 4 opposite to the imaging element 9 (the left side of the reference plane 4 in FIG. 9). In other words, as shown in FIG. 10, when the object material 5 is positioned at the left side of the reference plane 4, the object material 5 is focused at point B' on the left of point A' and an image having points b and b' replaced those of FIG. 9 is projected onto an imaging plane. This means that the points b and b' only come to positions reverse to each other when the object 5 is spaced apart from the reference plane 4 by .DELTA.L toward the imaging element 9 (FIG. 9) relative to a case when the object 5 is spaced apart from the reference plane 4 on the side opposite to the imaging element 9 by the same distance .DELTA. L (FIG. 10), and the double images actually projected onto the imaging plane are the same. Therefore, when the object 5 is located on the opposite side of the reference plane 4 from the imaging element incorrect distance information results, thereby reducing the precision of the imaging device.
The problem especially influences measurement precision when the object 5 is a moving target. For example, when the imaging device is used as an imaging device for measuring the distance to a robot, the establishment of the reference plane is difficult, resulting in difficulty in the measurement. If the reference plane is set at a position sufficiently far in order to avoid this problem, the change rate of .DELTA.d relative to .DELTA.L decreases, which reduces measurement precision. FIG. 12 shows the measurement result of the difference between the double images which are measured where the reference plane is set sufficiently far from the imaging plane and in a case where the reference plane is set in the neighborhood of the imaging plane, for an object (robot) which is spaced by .DELTA.L from the reference plane. As shown in the figure, the length difference .DELTA.d.sub.5 between the double images (b.sub.5, b'.sub.5) of the object in a case where the reference plane is set sufficiently far from the imaging plane is fairly small compared with the length difference .DELTA.d.sub.4 between the double images (b.sub.4, b'.sub.4) in a case where the reference plane is set in the neighborhood of the imaging plane. In this way, in order to increase the change rate of .DELTA.d relative to .DELTA.L and keep a high measurement precision, the reference plane should be set in the measureable range and this results in a problem with measurement precision.